After an oil or gas well has been drilled and a pay zone has been found, it is typically perforated to extend flow paths from the well into the pay zone(s) of interest. There are numerous stimulation procedures which enhance the production of the zone into the borehole. These procedures include treating the zone with various fluids including a procedure which props open the fractures in the zone to thereby improve fluid flow into the borehole. A fracture is often initiated by packing off the well above and below the perforations, and subjecting the perforations and the formation to hydraulic pressure by raising the pressure sufficiently to cause fracturing, and thereafter relieving the fracture. It is a procedure which is only inferentially analyzed from the surface. Measurements can be taken along the borehole to lead to estimates, perhaps even accurate quantification of fracturing parameters including measurement of the vertical fracture height. One technique used is the injection of a radioactive isotope tracer for the purpose of measuring the fractures in the region of the wellbore.
By definition, a tracer placed in the formation provides an appropriate radioactive emission which can be detected with a suitable detector in the borehole. But, this detected signal is always obscured by the background radiation associated with that particular formation. If a single tracer is used, some data can be obtained dependent on the location of the tracer in the fracture fluid. In this regard, it is generally possible to provide soluble tracer elements, including those which are selectively soluble in water or oil but not both. Also, particulate tracer elements can be placed in the formation through the use of encapsulated particles which are intended to behave like particulate sand in the formation. In any case, gamma spectroscopy techniques involve making measurements of gamma ray energy in selected windows of the spectrum to enable presentation of a log which will show appropriate radiation levels from injected radioactive isotope tracers in the formation.
Typically, this procedure occurs after making perforations through the casing, adjacent and surrounding cement layer, and into the formation. The casing is a shield of relatively dense, energy absorbing materials around the locus of the gamma responsive device placed in the borehole. The selected tracer isotope is chosen in part for the energy levels of photopeaks of the gamma emissions therefrom and in part based on the relative half life. For instance, it is possible to use a tracer with a half life of just a few days; other tracer isotopes provide half lives which are as high as sixty days (for .sup.124 antimony) and even higher. Obviously, there are longer isotope half lives, but they are typically not chosen for a variety of reasons. The tracer is normally injected with the fracture fluid and proppant. The fracturing fluid must be mixed at the surface before it is delivered into the formation. In most instances, the well known fracture fluid is mixed at the surface (it is primarily water plus selected solids) and it is mixed either on a continuous or batch basis, density is checked, and the fracture fluid is then pumped through high pressure pumps into the formations of interest. During this, it is desirable to inject the tracer element. Radioactive isotope detection is dependent on the concentration and the half life of the tracer. If, for instance, the tracer .sup.198 Au is used, it has a half life of only 2.7 days, and it must be quickly measured to provide a calibration standard to take into account its relatively short half life. Obviously, the quantity of tracer placed in the fracture fluid must also be determined. Thus, calibration for the measurements must be obtained at the surface so that suitable, useful and correct standards are available for making the later measurements, particularly preliminary to performance of the fracture job. It may not be known precisely in advance how much of the fracture fluid must be mixed, and one may equally be ignorant of the actual quantity of fracture fluid delivered into the formation. The size of the job can be estimated; the actual fluid injected in altogether a different measure.
Through the use of an exemplary radioactive isotope tracer, a first formation can be tested. If there is another formation perforated from the same well, it is desirable to use a different tracer for that stage. Accordingly, a second or another radioactive isotope tracer may be mixed for another slug of the fracture fluid. This might be tested with a different tracer; and if the first formation is tagged with the tracer .sup.198 Au, an alternate might be .sup.46 Sc. These are particularly desirable in a common test because the peak gamma radiation for gold is found at 412 KeV while scandium has peaks at 889 and also 1121 KeV.
To further complicate the foregoing, the initial fracturing proppant can be delivered with a first isotope and the last portions delivered can be tagged with a different tracer. This will help evaluate the proppant placement in the fractured formation with the view that the initial delivery of fracture proppant (presumably sand) is more deeply placed in the formation than the last delivered sand. Certain of these advantages have been set forth in some detail by the inventor of the present disclosure in the July 1989 issue of Petroleum Engineer. As detailed in that article, there are numerous ways to use single or multiple radioactive isotope tracer elements which are detected by gamma spectroscopy wherein the log interpretation provides some information regarding the success of the hydraulic fracturing procedure.
It is very helpful to carry out tracer injection subject to calibration. The apparatus of the present disclosure is directed to this. It is particularly useful because it is installed at the surface. Routinely, the fracture fluid is mixed at the surface and is delivered through high pressure pumps into the well borehole. At the surface, this involves the use of several large trucks to deliver the solids and fluid, and large blenders with one or more truck mounted pumps. Typically, they are connected on a common manifold system. In turn, this cooperates with the blender to provide connection from the manifold into the borehole for delivery of the mixed fracture fluid.
This mixing routine accomplishes delivery of a fracture fluid having a specific weight and volume. Typically, hydraulic fracturing treatments are carried out at specified pump pressures, flow rates and surface conditions which can be readily measured. It is in that context that the present disclosure enhances the mixing of the fracture fluid to provide the precise and controlled addition of a radioactive isotope tracer in the fracture fluid to assure proper mixing. The present apparatus thus incorporates a scintillation detector positioned adjacent the mud flow line prior to injection into the well to measure the radiation level and hence the relative quantity of tracer injected in relation to the fracture fluid flow rate. A supply of tracer material is provided and is delivered by a pump into the mud flow line. The pump is an adjustable speed, adjustable flow pump. Accordingly, it provides an output which is sufficiently high in pressure to overcome the back pressure prevailing in the fracture fluid flow line. Fracture fluid is measured by directing that flow through a densitometer and flowmeter. All of this data is delivered to a CPU which forms continuous calculations to determine the flow rate, thereby enabling proportioning of the tracer to the fracture fluid.
The present apparatus can be duplicated for injection of two or more different tracers sequentially or simultaneously into the fluid, etc. Moreover, the calibration of the rates at which the tracer elements are added enables the subsequent testing and measuring of the fracture height along the formation adjacent to the borehole. Measurements can be obtained through the use of gamma spectroscopic measuring devices lowered into the borehole after injection of the tracers. A method of injecting a fracture fluid into a formation through a well borehole is also set forth particularly featuring incorporation of tracer elements.